1. Field of the Invention
The present invention relates to a non-contact communication system in which a radio wave transmitted from an interrogator is received by a responder and the responder produces electric power from the received radio wave and in addition reproduces data from a modulated signal carried by the received radio wave in order to transmit, in reply to the received data, reply data back to the interrogator.
2. Description of the Prior Art
Conventionally, as a non-contact communication system, radio-frequency tags (RF tags) and ID cards are known that acquire electric power from a radio wave transmitted from an antenna in order to transmit the data stored in themselves. Such systems are used, for example, with the lift facilities at a skiing ground, with the ticket examination equipment at a railway station, and for sorting of baggage in general.
Such RF tags and ID cards are formed as a non-contact card that has a nonvolatile memory and a transmitter/receiver unit incorporated therein but that has no power source such as a battery. Such a non-contact card operates on the electric power it produces from a radio wave (radio-frequency modulated signal) it receives. Moreover, such a non-contact card communicates data with its communication partner by using a radio wave, and thus offers the advantage of non-contact data communication.
In such a non-contact communication system, a non-contact card is used, for example, as a responder. The responder receives a radio wave transmitted from an interrogator and acquires electric power from the received radio wave. Therefore, conventionally, the interrogator needs to be kept transmitting all the time a radio wave for communication that carries electric power sufficiently strong to permit the responder to operate at a relatively remote location. Thus, in this non-contact communication system, even when no responder is present within the range in which communication is possible, the interrogator needs to be kept transmitting the radio wave for communication, and this clearly is a waste of electric power.
Moreover, in this non-contact communication system, it is undesirable in the first place to keep the interrogator transmitting all the time a radio wave that carries electric power sufficiently strong to permit the responder to operate, because this may cause interference with the operation of other electric equipment, or may have adverse effects on human bodies.
An object of the present invention is to provide a non-contact communication system in which an interrogator consumes less electric power to transmit a radio wave.
Another object of the present invention is to provide a non-contact communication system in which the radio wave that an interrogator transmits when it is not communicating with a responder does not cause interference with the operation of other electric equipment nor have adverse effects on human bodies.
To achieve the above objects, according to one aspect of the present invention, in a non-contact communication system provided with a first communication unit for radiating a radio wave for communication and a second communication unit for communicating with the first communication unit, the first communication unit generates a radio wave for detection weaker than the radio wave for communication so that the first communication unit, by detecting a predetermined change in the radio wave for detection, recognizes that the second communication unit is present within a range communicable with the first communication unit and then starts transmitting the radio wave for communication.
According to this configuration, when the second communication unit is not present in the range communicable with the first communication unit and thus neither of them is performing communication operation, the first communication unit transmits a radio wave weaker than the radio wave it transmits for ordinary communication. Thus, less electric power is consumed and accordingly higher energy efficiency is attained than in a conventional system that requires that a strong radio wave for communication be kept transmitted all the time.
Moreover, when the second communication unit is not present in the range communicable with the first communication unit and thus neither of them is performing communication operation, the first communication unit transmits a radio wave weaker than the radio wave it transmits for ordinary communication. Thus, as long as no communication operation takes place, there is less possibility of interference with other electric equipment or adverse effects on human bodies.
Moreover, in the non-contact communication system according to the present invention, the first communication unit transmits alternatively the radio wave for communication or the radio wave for detection by using a single transmission means shared between those two radio waves. Using a single transmission means to transmit the radio wave for communication and the radio wave for detection eliminates the need to provide separate transmission means to transmit the radio wave for communication and the radio wave for detection. Thus, the first communication unit used here can be obtained simply by making a few modifications to a conventional configuration thereof, and thus does not require a larger-scale configuration.
Moreover, in the non-contact communication system according to the present invention, repeated activation and deactivation of the second communication unit occur at regular intervals because the radio wave for detection transmitted from the first communication unit is so feeble as to be equal to critical strength that divides between activation and deactivation of the second communication unit when the second communication unit is present within the range communicable with the first communication unit. The non-contact communication system is provided with a detecting means for detecting variation of amplitude in the radio waves transmitted from the first communication unit resulting from such repeated activation and deactivation of the second communication unit, a checking means for checking whether the second communication unit is present within the range communicable with the first communication unit or not on a basis of an output from the detecting means, and a control means for controlling the first communication unit to transmit the radio wave for communication on a basis of an output from the checking means.
According to this configuration, the radio wave for detection generated by the first communication unit when the second communication unit is out of the range communicable with the first communication unit is so feeble relative to the radio wave for communication as to generate electric power of critical strength that divides activation and deactivation of the second communication unit, and thus the second communication unit is activated and deactivated repeatedly at regular intervals. This causes variation to occur at regular intervals in the impedance with which the tuning circuit of the first communication unit is loaded. The first communication unit, by detecting this variation occurring at regular intervals, recognizes the presence of the second communication unit. Moreover, the second communication unit does not require any circuit other than the one which performs communication to make the first communication unit recognize the presence of the second communication unit, and thus the second communication unit having the same configuration as in a conventional system can be used in the non-contact communication system according to the present invention.
Moreover, in the non-contact communication system according to the present invention, the second communication unit has a signal generating means for generating a reply signal when the second communication unit catches the radio wave for detection so as to make the first communication unit recognize that the second communication unit is present within the range communicable with the first communication unit.
The second communication unit has a circuit that can operate on the insufficient electric power obtained from a radio wave that is generated by the first communication unit so as to be weaker than the radio wave for communication. In addition, this circuit keeps the weaker radio wave modulated at a fixed frequency all the time. Thus, when the first communication unit is made to recognize that the second communication unit is present in the range communicable therewith, the first communication unit can be fed with a signal that does not depend on the characteristics of the circuit provided within the second communication unit for achieving communication.
According to another aspect of the present invention, in a non-contact communication system provided with a first communication unit for radiating a radio wave for communication and a second communication unit for communicating with the first communication unit, a mechanical or optical sensor is provided within the first communication unit so that the first communication unit starts transmitting the radio wave for communication when the first communication unit recognizes that the second communication unit is present within the range communicable with the first communication unit. This makes it possible to reduce the electric power consumed when no communication takes place between the two communication units, and eliminate the possibility of serious interference with other electric equipment or adverse effects on human bodies.